Multi-channnel rf energy delivery with coagulum reduction

ABSTRACT

A system for efficient delivery of radio frequency (RF) energy to cardiac tissue with an ablation catheter used in catheter ablation, with new concepts regarding the interaction between RF energy and biological tissue. In addition, new insights into methods for coagulum reduction during RF ablation will be presented, and a quantitative model for ascertaining the propensity for coagulum formation during RF ablation will be introduced. Effective practical techniques a represented for multichannel simultaneous RF energy delivery with real-time calculation of the Coagulum Index, which estimates the probability of coagulum formation. This information is used in a feedback and control algorithm which effectively reduces the probability of coagulum formation during ablation. For each ablation channel, electrical coupling delivers an RF electrical current through an ablation electrode of the ablation catheter and a temperature sensor is positioned relative to the ablation electrode for measuring the temperature of cardiac tissue in contact with the ablation electrode. A current sensor is provided within each channel circuitry for measuring the current delivered through said electrical coupling and an information processor and RF output controller coupled to said temperature sensor and said current sensor for estimating the likelihood of coagulum formation. When this functionality is propagated simultaneously through multiple ablation channels, the resulting linear or curvilinear lesion is deeper with less gaps. Hence, the clinical result is improved due to improved lesion integrity.

BACKGROUND OF THE INVENTION

[0001] Radio frequency energy may be used to treat certain cardiacabnormalities, such as fibrillation, by ablating caridiac tissue. Radiofrequency energy is delivered by RF generators in two phases: (i) the“ramp up” phase in which a relatively high amount of power is deliveredto the ablating electrode until a desired set temperature is sensed bythe thermocouple or thermistor, and (ii) the “regulation” phase in whichpower is still being delivered but regulated at a lower level tomaintain the desired set temperature. This target temperature ispredetermined by the operator, and is generally 50° to 55° C. forablation of cardiac tissue.

[0002] Most RF generators have software modules which run simultaneouslyon portable computers during RF energy delivery to log the ablationepisode. Typically, the parameters logged are sensed impedance, powerdelivered, as well as tissue temperature sensed by either thermistors orthermocouples. Currently, this information is typically used forpost-procedural review.

[0003] The challenge in RF ablation of cardiac tissue is to create deeplesions in the cardiac tissue while avoiding coagulum formation. Itfollows that RF energy must be delivered efficiently into the tissue,and not delivered and lost into the blood medium. Current methods andsystems are not adequate to assure that RF energy is deliveredefficiently to cardiac tissue during an ablation procedure.

[0004] Prior studies in the delivery of RF energy have shown that whenelectrode-tissue contact is intermittent, the impedance value fluctuatesand the power delivered also has to rapidly adapt in order to reach ormaintain the target temperature. The rapid alternating impedance valuestherefore cause the output power waveform to also fluctuate rapidly. Ifthe rise-time of the RF power waveform is sharp, and the noise modulatedonto the RF source signal has high enough amplitude, it may be conducivefor coagulum formation because it may undesirably approximate thecoagulation waveform used by electro-surgical units. Therefore, thereremains a need for systems and methods for performing RF ablationwherein effective contact with target cardiac tissue is assured toachieve deeper lesions and reduced coagulum formation.

[0005] The methods and systems of the current invention provideefficient delivery of radio frequency (RF) energy to cardiac tissue withan ablation catheter, thereby yielding consistently effective RFablation procedures and improved patient outcomes.

SUMMARY OF THE INVENTION

[0006] The methods and systems of the current invention deliver RFenergy to cardiac tissue simultaneously through a series of channels ina manner that is designed to minimize the risk of an ineffectiveablation procedure due to coagulum formation. The methods and systemsutilize an information processor and RF output controller to carefullycontrol the rate and amount of RF energy delivered from an RF generatorto the cardiac tissue being ablated to improve the effectiveness of anablation procedure. The information processor and RF output controllerassures that RF energy is increased gradually during the initial ramp-upphase. Furthermore, the information processor and RF output controllerregulates delivery of RF energy during the ablation episode usinginformation gathered from a series of sensors that are delivered to thesite of ablation, preferably as part of an ablation catheter. The seriesof sensors include a series of temperature sensors and/or a multiplicityof current sensors. This feedback-control assures that propertemperature is maintained at the site of ablation and provides theability to abort an ablation procedure if effective tissue contact isnot established or maintained throughout the ablation procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The novel features believed characteristic of the invention areset forth in the appended claims. The invention itself, however, as wellas the preferred mode of use, further objectives and advantages thereof,is best understood by reference to the following detailed description ofthe embodiments in conjunction with the accompanying drawings andAppendix, wherein:

[0008]FIGS. 1A AND 1B are schematic diagrams of certain embodiments ofthe information processor and RF output controller and system of thecurrent invention (FIG. 1A), and user interface (FIG. 1B) for theinformation processor and RF output controller.

[0009] FIGS. 2A-B show catheter arrangements for efficient ablation;

[0010]FIGS. 3 and 4 show schematic block diagrams of an informationprocessor and RF output controller in accordance with the invention, forregulating delivery of RF energy to cardiac tissue through an ablationcatheter;

[0011]FIGS. 5A and 5B provide flow diagrams for the temperaturemeasurements, and FIG. 5C is a block diagram illustrating real timeanalog computation of voltage impedance and power;

[0012]FIG. 6 shows a schematic diagram of temperature regulationcircuitry used to regulate RF energy based on temperature readings.

[0013]FIG. 7 is a block diagram showing the regulation of delivery of RFenergy by an information processor and RF output controller according toone embodiment of the current invention that regulates current deliveredto each ablation electrode of a series of ablation electrodes,separately using digital logic.

[0014]FIG. 8 shows a record of a typical ablation episode using themethods and procedures of the current invention; and

[0015]FIG. 9 is a graph of logistic function with estimated probabilityof coagulum as the Dependent Variable, and C.I. as the PredictorVariable. FIGS. 10A and 10B show representative scattergrams of coagulumindex values from two RF ablation patient cases. FIG. 10A shows resultsfrom a patient study when gradual power delivery was not applied andmaximum power was set at 50 W. FIG. 10B shows results from a patientstudy using systems and methods according to the current invention wheregradual power delivery was applied for each ablation episode and maximumpower of the RF generator was set at 30 W.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0016] The methods and systems of the current invention utilize a novelinformation processor and RF output controller, also called amulti-channel RF ablation interface herein, to regulate delivery ofradio frequency (RF) energy from an RF generator, also called an RFenergy source herein, to cardiac tissue via an electrical couplingconnected to a series of ablation electrodes of an ablation catheter.The information processor and RF output controller assures that energyis delivered in a gradually increasing manner during an initial ramp-upphase to an ablation temperature set point, and at a rate thereafterthat is feedback-regulated to maintain the set-point temperature of thecardiac tissue at the site of ablation. Preferably, the temperature setpoint is selectable by a user. The delivery of energy is also preferablyfeedback-regulated by other parameters such as impedance, current,and/or power delivered to the ablation catheter to assure that effectivecontact between the ablation electrode and the cardiac tissue ismaintained.

[0017] The information processor and RF output controller of the currentinvention are capable of delivering energy to each electrode of theseries of ablation electrodes independently. In certain preferredembodiments, described herein, the information processor and RF outputcontroller uses analog methods for information processing and pulsewidthmodulation for RF energy control.

[0018] In preferred embodiments, the information processor and RF outputcontroller is capable of delivering RF energy to the electrodes of theseries of electrodes in any order or combination using methods describedherein. Preferably, a user can select the electrode, or combination ofelectrodes, to which the information processor and RF output controllerwill deliver energy.

[0019] As shown in FIG. 1A, the described information processor and RFoutput controller 100, also referred to herein as a multi-channel RFablation interface, is intended to make cardiac lesions in the humanheart in conjunction with commercially available radio-frequency (RF)lesion generators (RF generators) 150 and ablation catheters 160, suchas those manufactured by Cardima. The interface regulates RF energydelivery from the RF generator 150 to the ablation catheter 160 bytemperature feedback using readings of thermocouple sensors 162 embeddedin the catheters 160, as well as by other parameters such as impedanceand differential impedance. Electrical communication between theinformation processor and RF output controller and the catheter occursvia an electrical coupling 170. The feedback regulation functions tomaintain the electrode temperature near the preset temperature value,and to assure that effective contact between ablation electrodes 164 andcardiac tissue has been maintained for effective transmission of energyfrom the electrodes 164 to the cardiac tissue.

[0020] The general design features of the multi-channel RF ablationinterface (i.e. the information processor and RF output controller) ofthe current invention include an operating RF frequency range of about470 to about 510 kHz; multiple, preferably eight (8), regulatedelectrode channels; maximum power RF energy input of about 100 watts;maximum power RF energy output for each channel of 30 Watts; and afunction that provides gradually power delivery at start-up. Asdescribed below, preferably the power for each channel is typically setat about 25 to 35 watts, most preferably about 30 watts. The informationprocessor and RF output controller is typically capable of receivingreal-time temperature monitoring information from sensors 162 on theablation catheter 160, and compares this information with the userdefined set temperature. This temperature information is used to controlthe titration of RF energy to reach and maintain the set temperature, orto shut off RF energy delivery if a certain over-temperature cutoff isreached. The information processor and RF output controller alsocalculates real-time impedance and output power based on measurementssensed from the circuitry, then compares this calculated information touser set limits, wherein if a limit is exceeded, delivery of energy isterminated. Preferably the information processor and RF outputcontroller 100 is capable of receiving and processing this informationfor each output channel of the circuitry. The information processor andRF output controller may use analog or digital methods for receiving andprocessing monitoring information from the sensors. In a preferredembodiment real-time analog data acquisition and computation methods areused.

[0021] The information processor and RF output controller and/or the RFsource has the ability to deliver RF energy in a gradual manner whenenergy delivery is initiated. That is, either in a manual, or preferablyan automated manner, upon initiation of delivery of RF energy to anablation electrode, power is initiated at a level that is below themaximum power level used to attain a temperature set point for thecardiac tissue being ablated. Power is then gradually increased over aduration of about 8 to 15 seconds, preferably 10 seconds, typicallyuntil it reaches the maximum power. For example, but not intended to belimiting, when using the Radionics RFG-3E generator in the manual mode,power may be commenced with a setting of 10 watts, and then graduallyincreased within 10 seconds by adjusting the power knob on the RFgenerator to reach a set temperature of 50° C. while not overshooting amaximum of 30 watts, all the while maintaining total RF delivery time at60 seconds. Rather than a manually controlled mode, the preferredinformation processor and RF output controller and RF output controllerof the current invention, as described below in more detail, graduallyincreases power automatically upon initiation of RF energy delivery.

[0022] As shown in FIG. 1B, the information processor and multichannelsimultaneous RF output controller typically contains a user interfacecontaining a series of displays 105 and 110, and adjustment knobs 115,120, 125, 130, 135 to facilitate monitoring and control of theparameters described above. For example, the user interface may containa display of parameter values 105, and may preferably contain a separatethermocouple digital display 110.

[0023] The user interface typically contains a series of adjustmentknobs 115, 120, 125, 130, 135 to facilitate setting values for theparameters described above. For example, the information processor andRF output controller typically includes an ablation temperature setpoint control 115 and over-temperature set point control 120. Typicallythe ablation temperature set point control 115 has a range of from about50° C. to about 70° C., and the over-temperature set point control 120has a range from about 55° C. to about 75° C. Additionally, theinformation processor and RF output controller preferably can determineimpedance and differential impedance, typically measures power outputand includes a power limit adjustment knob 125. Preferably, theinformation processor and RF output controller has an impedance limitcontrol 130 which typically can be set in the range from about 50 toabout 1000 Ohms. Additionally, the information processor and RF outputcontroller preferably has a differential impedance set point control 135from 10 to 300 Ohms.

[0024] Finally, the information processor and RF output controller userinterface may contain a fault status indicator 140 which may project anytype of signal detectable by a user if the information processor and RFoutput controller detects a parameter value that exceeds a preset limit.For example, the fault status indicator may be triggered if thetemperature of the cardiac tissue exceeds a maximum temperature set bythe user. The fault status indicator may project a visual or auditorysignals. In certain preferred embodiments, the user interface includes areset switch which resets the fault status indicator.

[0025] The user interface on the information processor and RF outputcontroller may have one or more of the following additional features, asdescribed in more detail in the specific embodiment disclosed below:

[0026] 1. an ablate/pace mode select switch to switch between ablationand electro-cardiogram recording modes;

[0027] 2. ablate, RF active and pace indicator LEDs;

[0028] 3. a bipolar pacing stimulator selector switch;

[0029] 4. a parameter display pushbutton switch;

[0030] 5. an illuminated on/off electrodes select switch; and

[0031] 6. a real-time parameter data collection for post processing anddata analysis in commercial software programs such as, but not limitedto, LabView and Excel formats.

[0032] As mentioned above, the information processor and RF outputcontroller of the current invention regulates delivery of RF energy froman RF energy source through multiple channels simultaneously to cardiactissue. The primary functional building block of all radio frequency(RF) energy sources developed for tissue ablation is an electroniccircuit called an oscillator which generates sinusoidal waveforms atparticular operating frequencies. This waveform is consequentlyamplified to deliver the required wattage required for tissue ablation.The operating frequency of this RF oscillator typically is within therange of 470 to 510 kHz. The quality of the oscillator and ancillaryelectronics design impinges on the stability of the resulting operatingfrequency. Hence, this operating frequency may “drift” slightly if theoscillator design is unstable. Typically, this frequency jitter hasimperceptible influence on the resulting tissue lesion. However, certainRF oscillators or associated electronics systems generate and deliver askewed or distorted sine wave signal that has spurious noise spikesand/or harmonics riding on top of it. Such “noisy” and skewed RFwaveforms may result in undesirable noise artifacts may have thepotential of promoting coagulum formation if they are present during theablation process. Therefore, it is desirable for the current inventionto use an RF source which produces a relatively pure and stable sinewave, preferably as pure and stable a sine wave as possible.

[0033] As described above, the information processor and RF outputcontroller is connected to and regulates RF energy delivered to multipleelectrodes arranged in various configurations at the distal end of acatheter. In catheter ablation, electrodes of the catheter deliver theRF current into biological tissue. This RF energy in turn heats thetissue by causing ionic friction within the tissue and fluid mediumencompassed by the electric field. When monitored, this temperature risecaused by the conversion of electrical to thermal energy can be used asa guide in RF catheter ablation. Its measurement is facilitated by theplacement of thermal sensors, either thermocouples or thermistors,underneath or juxtaposed with the ablative electrodes. Not only can thesensed temperature be used to ascertain the quality of electrode-tissuecontact and predict lesion size, it can also be utilized by the RFgenerator as a feedback signal to automatically regulate the outputpower to arrive at or maintain a temperature set-point predetermined bythe end-user.

[0034] Many ablation catheters are known in the art and can be used withthe systems and methods of the current invention. Typically, cathetersfor use with the current invention have multiple electrodes and thermalsensors in close proximity to these electrodes, as discussed above.Furthermore, preferred catheters allow relatively higher electrodecurrent densities which allow lower maximum RF generator power settings,such that effective ablation can be performed at 35 W, and morepreferably 30 W, rather than 50 W.

[0035] An example of a preferred catheter (i.e., the CARDIMA Revelation™TX 3.7 Fr catheter) for use in the current invention is illustrated inFIGS. 2A-2B. The catheter was developed for right atrial linear MAZEablation, and has eight electrodes with thermocouples located in betweenthe electrodes, to accurately sense localized tissue temperature at theablation site. This preferred catheter has eight 6 mm coil electrodeswith 2 mm inter-electrode spacing, and 8 thermocouples located proximalto each electrode in the inter-electrode spaces. A 9 Fr steerableguiding catheter called the Naviport™ may be used in conjunction withthis catheter to aid in placement. Experience with the 3.7 Fr REVELATIONTx microcatheter has shown that it is successful in creating transmurallesions narrower and with smaller surface area than those created bystandard 8 Fr ablation catheters.

[0036] In order to switch between each of the multiple electrodes andtheir corresponding thermocouples or thermistors, manual switchboxesinterfacing multi-electrode catheters to single-channel RF generators,as well as automatic sequencing multi-channel RF energy generators havebeen developed and are now available in the marketplace. Theseswitchboxes and multi-channel RF generators deliver RF energy to theseelectrodes in a consecutive, sequential fashion. In addition, there arealso newer, higher power (e.g., 150W) RF generators which deliver RFenergy simultaneously to multiple electrodes. These latter systemsdiffer in design by how RF energy “is split” among the various electrodechannels. This present invention presents a multichannel RF ablationsystem which uses pulse width modulation to govern the amount of RFenergy being delivered at each channel, incorporating temperaturefeedback information per channel as well as from neighboring channels.

[0037] With these general features of a system for the delivery of RFenergy to cardiac tissue according to the present invention, a specificembodiment is diagrammatically illustrated in FIGS. 3 and 4. Thedescribed embodiment provides a specific multi-channel RF ablationsystem with the general features illustrated in FIGS. 1A and 1B. Themultichannel information processor and RF energy controller provides upto eight channels (switch selectable) of precise RF energy to thecatheter's electrodes as well as displays the tissue temperature andimpedance in real time. Measurement of the RF power delivered to thetissue, RF current, and RF voltage, as well as the differentialimpedance for each of the ablation elements, is also provided. Allsignals are available for computer monitoring or optionally displayedvia front panel digital meters. The system incorporates a medical gradepower supply approved by the international safety agencies. This powersupply can be used for various line voltages and frequencies without anymodification. The system is designed to handle up to 100 watts of inputpower RF energy. Utilizing an analog computer unit (ACU), the systemcontinuously monitors and adjusts the precise RF energy delivered toeach electrode.

[0038] The following are features of the pulse width modulationimplementation for the system: (1) soft start power-on operation; (2)compensation for the lag in thermocouple response time; and (3) PWMsynchronization for all eight channels.

[0039] Over-temperature detection is provided for each channel of thesystem. RF energy is latched off for the entire system if anover-temperature condition is detected. Operation is resumed by powercycling or pushbutton reset. Open thermocouple detection inhibitsoperation of only the faulty channel. Operation is resumed automaticallywhen the fault is cleared. The system is designed to comply with therequirements and standards of international electrical safety codes. Itutilizes isolated circuits for all patient connections to insure patientsafety even with failed components. This applies to both thethermocouple amplifiers, and the RF output circuitry. Theover-temperature cutoff limit is provided to cut off all power deliveredto the catheter in the event that any thermocouple reaches a presetover-temperature limit. Adjustment range for this function is from 55°C. to 75° C.

[0040] A front panel control and display unit is provided which allows auser to set a number of parameters. For example, the front panel controland display can be used to set the maximum power value sent to any oneelectrode (Adjustment range: 1-30 watts). The impedance cutoff circuitrymonitors each channel individually and will cause the power delivery tobe interrupted from a given electrode when that electrode's impedancerises above a preset limit. The front panel control and display (one forthe entire unit) provide a control button or knob for setting theimpedance cutoff limit (Adjustment range: 50-1000 Ohms). Thedifferential impedance cutoff circuitry monitors each channelindividually and will interrupt power delivered to a given electrode ifthat electrode's impedance rises by a preset differential (above thelowest value during a given ablation run). The front panel control anddisplay provides a knob for setting the differential impedance cutofflimit (Adjustment range: 10-200 Ohms). In order to prevent an RFgenerator trip-out due to low impedance (as can occur when severalelectrodes are running in parallel simultaneously), an active impedancenetwork (dummy loads) are placed between the RF generator and theablation circuitry.

[0041] A mode switch (ablate/pace) is provided for switching betweenablation and electrocardiogram recording modes, as well as pacethreshold determination mode. Appropriate filtering is designed to allowrecording of electrocardiogram during ablation or pacing modes. Modes ofOperation:

[0042] (Mode 1) Used for catheters that utilize thermocouples betweenelectrodes (e.g., thermocouple 1 is proximal to thermocouple 2). Thesystem will monitor temperature on both sides of each electrode andregulate the temperature based upon the higher temperature, except forthe most distal electrode, which has only one nearest thermocouple.

[0043] (Mode 2) Used for catheters utilizing thermocouples either underor soldered directly onto each electrode.

[0044] The channel card functional block diagram (FIGS. 3 and 4) of thesystem 10 provide thermocouple inputs and patient isolation 12, pulsewidth modulator 14, power output RF control 16, analog computer andparameter measurement 18, impedance and differential impedance 20, faultlatch control 22, and fault status 28.

[0045] The common mode input filter is designed to handle high commonmode of RF energy level on the thermocouples. The isolation circuits,both the power supply and the thermocouple amplifiers, are designed toisolate the patient from the main power source circuitry by 2500 volts.

[0046] The pulse width modulator (PWM) 14 regulates the RF energy bycomparing the delivered RF power (computed by the analog computer) tothe preset value (PLIMIT). It also provides soft start for each channelcard as well as synchronization circuitry for all eight channels. Thesoft start is a safety feature active at power on that gradually rampsup the voltage to prevent spikes on the electrodes.

[0047] As shown diagrammatically in FIGS. 5A-B, the amount of energydelivered to the RF coupling transformer is directly proportional to thepulse width generated by the PWM circuitry based on the temperature feedback from the catheter's thermocouple. In the preferred example of anablation catheter for the current invention described above, eachchannel has a corresponding thermocouple (T/C) sensor which providestemperature feedback information at the tissue site immediately proximalto the electrode delivering RF energy. The RF output for each electrodeis modulated by a PWM chip on the channel card. The commerciallyavailable PWM device used is the Unitrode High Speed PWM ControllerUC3823, or the equivalent chip made by MicroLinear, ML4823. Temperatureinput signals sensed from neighboring T/C's are used to control thepulse-width modulator (PWM) outputs. The lower the input voltagecorresponding to an input temperature, the longer the “on time”duration. Conversely, the higher the input voltage corresponding to asensed input temperature, the shorter the “on time” duration.

[0048] The temperature regulation circuitry of this specific example isshown in more detail in FIG. 6. As mentioned above, each electrode 164has a corresponding thermocouple sensor 162 that provides temperaturefeedback information at the tissue site immediately proximal to theelectrode delivering the RF energy. Each electrode's RF output iscontrolled by a PWM circuit 180 located on each channel card.Temperature input signals sensed from neighboring thermocouples that areelectronically subtracted from each other to form a new pulse width thatwill control the amount of RF energy output. For example, FIG. 6illustrates the monitoring of both sides of electrode #5 and theresulting differential PWM that will control the RF circuitry for thiselectrode. As illustrated, digital logic, herein NAND gate 185 isemployed with inputs set by temperature thresholds taken fromthermocouples adjacent to the electrodes.

[0049] Safety features that isolate the external RF generator (couplingtransformers) from the power source are implemented both on the channelcard as well as common electronics board.

[0050] The voltage, current, impedance, and output power are calculatedby the analog computer unit (ACU) and the associated high precision RMSto DC converter circuitry. The information generated by the ACU iscrucial to the precise control and stability of the system. Thisprovides real-time monitoring of the catheter's parameters andstabilizes the preset temperature for a constant stream of energy inorder to create a clean and accurate lesion.

[0051] As shown diagrammatically in FIGS. 5A-B, this interface providesan impedance and delta impedance cutoff for each channel individually.This will cause the power delivery to be interrupted from a givenelectrode when that electrode's impedance rises above a preset limit.

[0052] Over temperature, open thermocouple, high impedance, and highdelta impedance detection circuitry are implemented into the design ofthe preferred example of an information processor and RF outputcontroller (i.e. the IntelliTemp system) described herein. Systemshutdown occurs for over temperature detection on any channel. Openthermocouple will inhibit operation on the affected channel only, normaloperation proceeds on remaining channels.

[0053] The following parameters are used for real time analogcomputation of voltage impedance and power according to the specificexample of an information processor and RF output controller describedabove:

[0054] Input parameters:

[0055] Sensed AC Voltage, V_(in), via secondary side of the inputtransformer.

[0056] Sensed AC Current, I_(in), mA, via precision non-inductiveresistor and associated circuitry.

[0057] Output parameters:

[0058] Computed RMS Voltage, V_(out), 100 mV/RMS representing 1 Volt, V.

[0059] Converted RMS Current, V_(out), 10 mV/RMS representing 1milliampere, mA.

[0060] Computed Impedance, Z_(out), 1 mV/RMS representing 1 ohm, Ω.

[0061] Computed RMS Power, P_(out), 100 mV/RMS representing 1 Watt, W.

[0062] Introduction:

[0063] The specific example of the information processor and RF outputcontroller illustrated in FIGS. 3-7 does not rely on digital circuitry(e.g., analog-to-digital (AID) converters, digital latches, registers,and a microprocessor) to determine sensed voltage, impedance, and power.Instead, it utilizes analog methods to provide real-time computation ofRMS output, voltage, current, impedance and power.

[0064] The building blocks for the real-time analog computer areillustrated in FIG. 5C and described in the following paragraphs.

[0065] 1. The primary building block for this analog computationcircuitry is the Analog Devices AD538 Real-Time Analog Computation Unit(ACU) which provides precision analog multiplication, division, andexponentiation. The first two mathematical operations are used, asfollows:

[0066] The ACU has this transfer function:

V_(OUT,ACU)=V_(y)(V_(z)/V_(x))

[0067] It should be noted that this V_(OUT,ACU) is not the overallV_(OUT) of the analog computation system; it is merely the output of theAD538 device used. V_(z) is a DC value that is an output parameter fromthe second set of building blocks mentioned below, the RMS-to-DCConverter. This DC value represents the RMS voltage (V) of the RF energybeing delivered at the electrode. Similarly, V_(x) is a DC value whichhas been converted from the RMS current (mA), of the RF energy beingdelivered at the electrode. This device also permits a scaling factor,V_(y), to be multiplied into the output transfer function. This scalingfactor is set at a value of 0.1, since the ratio of the primary tosecondary coils of the input transformer is 10. Since V_(z) representsvoltage, and V_(x) represents current, therefore V_(OUT,ACU) representsthe computed real-time impedance Ω.

[0068] 2. The secondary building blocks are two Analog Devices AD637High Precision Wide-Band RMS-to-DC Converters, which serve to computethe true RMS value of an incoming AC waveform, and represent this RMSvalue as an equivalent DC output voltage. The outputs of these units arefed as input parameters into the ACU discussed above, which alsosupplies a true RMS value of a signal that may be more useful than anaverage rectified signal since it relates directly to the power of theinput signal.

[0069] 3. The final building block is the Analog Device AD734 4-QuadrantMultiplier/Divider, which serves to multiply the DC value representingRMS Voltage, with the DC value representing RMS Current, to supply theproduct of these two terms, which is equivalent to Output Power, sinceP_(out)=V_(out)I_(out)(W, Watts).

[0070] 4. The outputs of V_(out), I_(out), Z_(out), and P_(out), arehence all calculated in real-time.

[0071] RF output per channel is governed by three inputs into a NANDgate (Motorola part number MC74HC10A):

[0072] i. The “on time” of the pulse-width modulator for that particularchannel.

[0073] ii. The “on time” of the pulse-width modulator for the channelimmediately proximal to the above-said channel.

[0074] iii. Power Limit Set-Point that is common for all channels. Thisis manually set with a control knob on the instrumentation front panel.

[0075] As an example, the functional schematic of the interactionbetween Channel 3 input and Channel 2 output in determining Channel 3output is shown in FIG. 7, where in the timing diagram of the Channel 3electrode output (lower right corner) there is a slight propagationdelay.

[0076] The PWM duty cycle is governed by an oscillator that is set by anoscillating frequency determined by a resistive and a capacitivecomponent. In the present embodiment, this frequency is set at 1.7 kHz.However, if the sensitivity of the feedback-response circuit needs to be“slowed down” to increase heat build-up in the tissue, this frequencycan be decreased.

[0077]FIG. 8 shows a typical ablation episode using the specificembodiment of the invention described above. Contact force is aparameter that has been measured experimentally in an in vitro settingto determine the quality of electrode-tissue contact; it has a highcorrelation (up to 97%) with temperature rise. Thus, when there isexcellent electrode-tissue contact, there is a regular flow of RF energytransmitted into the tissue that is converted into heat energy. Whenthis condition exists, the monitored tissue impedance and voltage isrelatively constant. Therefore, the measured tissue impedance is anotherkey parameter, because it is an indicator of electrode-tissue contact.

[0078] As described above, the information processor and RF outputcontroller of the current invention, as well as the systems and methodsof the current invention, are designed to maximize the efficacy of anablation procedure by minimizing coagulum formation. Not to be limitedby theory, these information processor and RF output controllers,systems, and methods take advantage of the following considerations.When tissue contact is good and stable, the impedance is relatively lowand constant. As a result, less RF energy is required to reach thedesired set temperature, with a shorter “ramp up” time and a lowerwattage required to maintain the set temperature. The risk of coagulumformation is low because RF energy is effectively transmitted into thetissue, and heat is generated within the tissue rather than at the bloodlayer.

[0079] Conversely, when electrode-tissue contact is intermittent, theimpedance value fluctuates and the power delivered also has to adaptrapidly in order to reach or maintain set temperature. This fluctuatingwaveform may be conducive for coagulum formation because the rapid backand forth switching between high and low impedance causes the outputpower waveform to approximates the coagulation waveform used inelectrosurgery.

[0080] When electrode-tissue contact is marginal or poor, impedance canrise rapidly thereby requiring more RF energy to be delivered in a fastresponse to achieve the same set temperature. In this last scenario,because of poor electrode-tissue contact, there is a high probabilitythat RF energy is lost into the blood layer surrounding the electrode,thus heating the blood rather than tissue and fostering coagulumformation. As coagulum forms on the electrode, impedance rises evenmore, hence bringing about a vicious cycle of climbing wafts andescalating thrombus formation. Hence, one has to terminate the powerdelivery immediately when there is a sudden impedance rise, and thecatheter should be withdrawn at this point to clean coagulum off theelectrodes.

[0081] The following example describes and illustrates the methods,systems, and devices of the invention. The example is intended to bemerely illustrative of the present invention, and not limiting thereofin either scope or spirit. Unless indicated otherwise, all percentagesand ratios are by weight. Those skilled in the art will readilyunderstand that variations of the materials, conditions, and processesdescribed in these examples can be used. All references cited herein areincorporated by reference.

EXAMPLE

[0082] A study was performed to analyze factors that affect coagulationformation during cardiac ablation, and to set parameters to minimizecoagulation formation during this procedure. More specifically, thestudy was performed, at least in part, to analyze the rate of RF powerdelivery through ablation catheter electrodes with respect to targettemperature set-points, and to determine its correspondence to coagulumformation.

[0083] This study was based on RF ablation data from 398 independentablation episodes derived from 15 patient cases randomly picked fromPhase II of the CARDIMA REVELATION™ Tx U.S. multicenter clinical trials.Patient entry criteria were symptomatic paroxysmal atrial fibrillation(PAF), refractory to at least 2 anti-arrhythmic drugs, with 3 PAFepisodes within the 30 day baseline observation period. In thismulticenter clinical protocol, the use of anti-coagulation agentsfollowed these guidelines for all patients receiving RF ablation:Coumadin OK was discontinued three (3) days prior to the procedure andlow molecular weight heparin was administered the day preceding theprocedure. At the time of the procedure, the international normalizationratio (INR) was checked to be<1.8, and a baseline activated clottingtime (ACT) value was obtained. An initial bolus of intravenous heparinwas administered, and continuously administered throughout the procedureto maintain an ACT of approximately 200 to 300 seconds. The ACTmeasurements were taken at 30 minute intervals until therapeutic levelswere achieved, then every 60 minutes for the duration of the procedure.Heparin administration was adjusted according to the ACT values.

[0084] RF ablation procedures were performed using the REVELATION Tx(CARDIMA, Fremont, Calif., U.S.A.) Microcatheter. This microcatheter haseight 6 mm coil electrodes with 2 mm spacing, and eight inter-electrodethermocouples. A 9 Fr CARDIMA NAVIPORT™ steerable guiding catheter wasused in conjunction with the microcatheter to aid in placement. If thetarget temperature was not reached, the duration to reach the maximalrecorded temperature closest to the target temperature was used instead.The RFG-3E RF generator (Radionics, Burlington, Mass., U.S.A.) was theRF source used for all procedures.

[0085] Software running on a computer connected to this generator wasused to record the time for attaining a pre-determined targettemperature, as well as the RF power and current at that time, for eachRF energy application. Measurements taken included the duration time(seconds), for attaining a pre-determined temperature set-point (i.e.,50° or 55° C.), and the power (watts) at that time. This was carried outfor each RF energy delivery episode corresponding to each electrode. Ifthe set-temperature was not reached, the duration to reach the maximalrecorded temperature closest to the set-temperature was used instead.After each linear ablation trajectory, the catheter was withdrawn fromthe steerable guiding sheath, and each electrode was visually inspected.The presence or absence of coagulum was noted on clinical data sheets,thereby providing a record for analysis with the RF deliveryparameters,(i.e. power, current, and duration to reach targettemperature) that were logged automatically by software.

[0086] Based on the study described above, a mathematical model was usedto calculate a value, the Coagulum Index, that provides insight into thelikelihood of coagulum formation during an ablation procedure, and thatis useful in setting parameters for an ablation procedure to minimizethe potential for coagulum formation. From this model, Coagulum Indexwas defined:

[0087] Coagulum Index=(W/t)/I²

[0088] Power=W (wafts)

[0089] Current=I (amperes)

[0090] Duration to reach Set Temperature=t (seconds)

[0091] The term on the right-hand-side of the equation, (W/t), is theslope or gradient of the power curve measured from the start of theablation episode (baseline) to the time that the target temperature(i.e. set point temperature) or maximum temperature is first reached inan ablation episode. The derivation of the Coagulum Index, which has nophysical units, is included in Appendix A.

[0092] Many dose-response relationships have been found to follow alogistic sigmoidal curve. Hence, the estimated probability of coagulumoccurring, P(coag), is modeled statistically by a logistic modeldescribed by Equation 1 below, where the logit risk of coagulum is thedependant variable and the coagulum index (C.I.) is the independent orpredictive variable. $\begin{matrix}{{{P({coag})} = \frac{^{\alpha} + {\beta \left( {C.I.} \right)}}{1 + ^{\alpha} + {\beta \left( {C.I.} \right)}}}\quad {\alpha = {- 5.2932}}\quad {\beta = 0.3803}} & \text{Equation~~1}\end{matrix}$

[0093]FIG. 9 shows the graph of this logistic model. With this model, athreshold value for coagulum index (C.I.) can be found to indicate ahigh probability of coagulum occurring.

[0094] In a series of 398 ablation episodes from a total of 15 patientstudies in the clinical studies described in this Example, it was foundthat the logistic model of risk of coagulum demonstrated a significantfit between Coagulum Index and the estimated percentage probability ofcoagulum occurring (p<0.001). Table I summarizes the finding that theestimated probability of coagulum formation increases significantly whenCoagulum Index increases. This analysis revealed a clear correspondencebetween Coagulum Index and coagulum formation. Furthermore, a distinctthreshold of Coagulum Index greater than or equal to 12 was established,beyond which coagulum formation is expected. Results of this studyshowed that coagulum could be reduced if the slope (W/t) was gentle.This was accomplished by gradually increasing the power delivered fromthe RF generator, as opposed to “cranking up the watts” at the verystart of an ablation episode. TABLE I C.I. 4  8 12 16 20 24 P(coag) % 210 32 69 91 98

[0095]FIGS. 10A and 10B show representative scattergrams of CoagulumIndex values from two RF ablation patient cases. This data supports theconclusion that the derived Coagulum Index value has pertinence andvalue in suggesting coagulum formation. The example depicted in FIG.10B, with Coagulum Index values less than 12, showed no coagulumformation. On the other hand, coagulum was observed in many of theenergy applications of FIG. 10A, especially for those having a CoagulumIndex greater than 12. For the energy applications in FIG. 10B, lowerCoagulum Indexes were obtained by gradually increasing power, incontrast to an immediate increase in power levels which was used for theenergy applications shown FIG. 10A. Furthermore, the maximum powersetting was reduced from 50 to 30 watts in FIG. 10B.

[0096] The clinical effectiveness was analyzed for linear ablationprocedures where coagulation formation was absent. During Phase I, powerdelivery was not controlled in a gradual manner for each ablationepisode and maximum power was set at 50W. During Phase II, ablation wasperformed using a gradual power delivery (as described below) andmaximum power was kept below 35W. As summarized in Table II, after 6months AF episodes were reduced in Phase II patient populations. Infact, the number of patients experiencing a greater than 50% reductionin AF episodes almost doubled when using the gradual power delivery andlower maximum power for each ablation episode. A significant increasewas also observed in the number of patients that no longer had any AFepisodes (100% reduction), from 30% in Phase I to 53% in Phase II. TABLEII % Reduction of AF Episodes Phase I Phase II after 6 months (N = 10)(N = 17) >50% reduction 4/10 Patients 13/17 patients (40%) (77%) 100%reduction (no AF 3/10 Patients  9/17 patients Episodes) (30%) (53%)

[0097] Thus, it appears that one mechanism for mitigating coagulumformation is to deliver RF power in such a way that the rise time of thepower, and hence temperature curve, is more gradual and consistent. Forexample, when using the Radionics RFG-3E generator, with a set maximumof 30 wafts, one should commence with a lower power setting of 10 waftsfor the about first 10 seconds, and then gradually adjust the knob onthe RF generator to the set maximum of 30 watts, while still maintainingtotal RF delivery time at 60 seconds. When this technique was applied,it decreased coagulum formation, as is evident by the data shown in FIG.10.

[0098] Specific characteristics of RF generators must be considered toobtain the gradual power rise described above. The IBI-1500T has 4user-selectable choices for controlling the power delivery ramp-upcurve. The Osypka 300 Smart and Cordis Webster Stockert have built-inalgorithms which appear to automatically regulate power delivery risetime in a gradual manner, the latter allowing the end-user to specify atemperature ramp-up time. And finally, the Medtronic Atakr has no useroverride controls for power delivery application. In comparison, theRadionics RFG-3E allows the user to manually increase power outputduring the delivery of RF energy. In the present embodiment of thisinvention, the output power setting for RF energy to be delivered at theelectrodes are user adjustable via the front panel knob (1-30 Watt). Alower power setting will increase the ramping time, since it takes alonger time to reach set temperature. An automatic algorithm whichcalculates the coagulum index (C.I.) in real-time can be incorporatedinto the information processor and RF output controller functionality sothat a visual or auditory signal can alert the end-user whenever therisk for coagulum formation is high, i.e. C.I. greater or equal to 12.Alternatively, the information processor can calculate the C.I. inreal-time and use this calculated value as information that is fed backto the RF output controller functionality so that the ablation episodecan be carried out with minimal probability of coagulum formation.

[0099] Excellent electrode-tissue contact is determined by a combinationof fluoroscopy, low initial impedance, and the quality of electrogramsduring the procedure. Results from the study reveal that excellentelectrode-tissue contact, in combination with gradual RF power deliveryto a maximum level of 30 to 35 watts, constitutes a sound prescriptionfor best practice of RF ablation with the least likelihood of coagulumformation at the electrode site. Bench testing of tissue ablation hasalso demonstrated that good electrode contact with the tissue results inlower RF power consumption required to reach set temperature. Lower RFenergy requirements in turn reduce the probability of coagulumformation.

[0100] The insights revealed in this example may be extrapolated toprocedures using other catheters for other RF ablation procedures aswell, and hence are presented here. The catheter MAZE procedure callsfor the creation of linear ‘barricades’ along anatomical trajectorieswithin the right atrium, using RF ablation to compartmentalize thechamber and ‘contain’ pro-arrhythmic electrical propagation.

[0101] Results of this study reveal the following considerationsregarding minimizing coagulum formation during cardiac tissue ablation.In ideal situations, it is possible to achieve satisfactory tissuecontact for all eight linear ablation catheter electrodes. However, thetechniques discussed below yield acceptable results in right atrial MAZElinear ablation procedures even when the anatomical or flow conditionsprevent optimal simultaneous contact of eight catheter electrodes.

[0102] a) Excellent contact should be established in as many lineararray electrodes as possible.

[0103] b) Low tissue impedance at ‘baseline’ is indicative of effectivecontact; some RF generators permit this to be sensed and displayed priorto actual ablation by emitting a small RF current to interrogate tissueimpedance at the ablation site.

[0104] c) Pacing threshold, if used as an indicator of contact, shouldbe reasonable (1-2 mA); threshold values above 4-5 mA most likelyindicate poor contact, and the catheter should be repositioned.

[0105] d) The sheath should be rinsed periodically (e.g. every 15minutes) with a standard heparinized saline solution bolus. Thisimproves contact by removing coagulum build-up on the electrodes andcatheter shaft. If possible, the catheter should be pulled out of theNaviport deflectable guiding sheath after each trajectory; theelectrodes should be wiped clean if needed, before re-introducing thecatheter into the Naviport.

[0106] In addition to achieving excellent electrode-tissue contact,reduced coagulum formation can be obtained by regulating the RF powersettings such that power is gradually increased and by setting thegenerator maximum power settings to 30W-35W with power monitoredcontinuously. The catheter should be repositioned as needed to maintainset temperature at a lower power level. It has been observed thatcoagulum formation is more evident when power required to maintain settemperature approaches 50 W. Conversely, coagulum formation is minimizedgreatly when power required is less than 35 W. This may be seen as achallenge when trying to reach set temperature. However, with excellentelectrode-tissue contact, desired set temperature can be achieved withas low as 7 W to 15 W of power delivery. In vivo animal studies haveverified deep, transmural lesions with these low power settings whenthere is sufficient electrode-tissue contact.

[0107] While there has been illustrated and described a preferredembodiment of the present invention, it will be appreciated thatmodifications may occur to those skilled in the art, and it is intendedin the appended claims to cover all those changes and modificationswhich fall within the true spirit and scope of the present invention.

Appendix A Mathematical Derivation of Coagulum Index, using DimensionalAnalysis of Physical Parameters Pertinent to RF Ablation

[0108] A mathematical model for distinguishing between coagulum ornon-coagulum formation on the RF-ablating electrode of the CardimaREVELATION Tx catheter was developed. This model was based ondimensional analysis of physical constants pertaining to the units forvarious logged parameters during RF ablation episodes, and was verifiedusing clinical data obtained as described in the Example section.

[0109] Definitions in S. L (System International) Units:

[0110] Mass=Kg [kilogram]

[0111] Length=m [meter]

[0112] Time=s [seconds]

[0113] Power=W [watts]=Kg*m²*s⁻³

[0114] Each single-electrode catheter ablation event has its own slopecalculated from a plot of Power (Y-axis) vs. Time (X-axis), frombaseline temperature (i.e., temperature of free-flowing blood in theheart=approximately 37° C.) to 50° C. In this analysis, this is theduration for the sensed temperature from a thermocouple to reach a settemperature, e.g., 50° C. If the set temperature cannot be reached, thenit is the duration for the sensed temperature to reach the maximumtemperature, for that ablation episode. $\begin{matrix}\begin{matrix}{{Slope} = {{Power}\text{/Time}}} \\{= {\left( {{Work}\quad {Done}\text{/}{Time}} \right)\text{/}{Time}^{2}}} \\{= {\left( {{Force}*{Displacement}} \right)\text{/}{Time}^{2}}} \\{= {\left( {{Mass}*{acceleration}*{Displacement}} \right)/{Time}^{2}}}\end{matrix} & \text{Eqn.~~[1]}\end{matrix}$

[0115] Dimensional Analysis of the units show that: $\begin{matrix}{{Slope} = {{{Kg}*m*s^{- 2}*{m/s^{2}}}\quad = {{Kg}*m^{2}*s^{- 4}}}} & \text{Eqn.~~[2]}\end{matrix}$

[0116] It follows that I/Slope is the reciprocal of Eqn. [2]:

1/Slope=Kg ⁻¹ *m ² *s ⁴  Eqn.[3]

[0117] Now we define electrical capacitance, C, in terms of itsfundamental units:

C=m ⁻² *Kg ⁻¹ *s ⁴ *I ^(2 [NIST])

Rearranging terms, C=Kg ⁻¹ *m ⁻² *s ⁴ *I ²  Eqn.[4]

[0118] Dividing both sides by I²:

C/I ² =Kg ⁻¹ *m ⁻² *s ⁴ =t/W  Eqn.[5]

[0119] Notice that Eqn. [3]=Eqn. [5]

[0120] Therefore, we can define capacitance as a function of the slopethat we obtain for each ablation episode:

C=I ²*(t/W)=I ²/(W/t)=I ²/Slope  Eqn.[6]

[0121] In the presence of an alternating current, impedance Z is definedas:

Z=1/(2πfC)  Eqn.[7]

[0122] where f=operational RF frequency

[0123] Substituting Eqn. [6] into Eqn. [6], we are able to defineCoagulum Index as follows:

Relative Impedance=k*(W/t)/I ²  Eqn.[8]

[0124] where k=1/(2πf), and is constant for a particular RF generator,assuming that the RF oscillator frequency, f, is stable and constant.Therefore, for practical purposes, the proportionality constant k isignored in the calculation since the same type of RF generator, theRadionics RFG-3E, was used throughout the study described in theExamples section. The results discussed in the Examples section showed aclose correspondence between this calculated value and the probabilityof coagulum formation at the ablation electrode site. Therefore, theterm Coagulum Index was given to this quantity. We therefore arrive at:

Coagulum Index=(W/t)/I ²

What is claimed is:
 1. A system for efficient delivery of radio frequency (RF) energy to cardiac tissue with an ablation catheter, said system comprising: (a) an RF generator; (b) an electrical coupling effective for delivering an electrical current from the RF generator through a multiplicity of ablation electrodes, arranged in a linear or curvilinear assembly at a distal section of the ablation catheter, to the cardiac tissue and a return path for the RF current through a reference electrode; (c) a multiplicity of temperature sensors each positioned in proximity to each of the multiplicity of ablation electrodes, said multiplicity of temperature sensors effective for measuring the temperature of cardiac tissue in contact with the multiplicity of ablation electrodes; and (d) an information processor and RF output controller effective for controlling the amount of RF power delivered through the electrical coupling to provide a gradual increase in RF power calculated in real-time during an initial ramp-up phase, and to limit the delivery of RF power through the electrical coupling based on the temperature of cardiac tissue in contact with the series of ablation electrodes, thereby reducing the likelihood of coagulum formation during delivery of RF energy to cardiac tissue.
 2. The system of claim 1, further comprising a current sensor effective for measuring current delivered through said electrical coupling, and a voltage sensor effective for measuring voltage delivered through said electrical coupling, wherein the information processor and RF output controller is capable of calculating RF power in real-time and terminating delivery of RF energy through the electrical coupling based on changes in measured current and voltage, and calculated power, and wherein the information processor and RF output controller provide RF energy simultaneously to all or any combination of the multiplicity of electrodes in a user-selectable manner.
 3. A system as recited in claim 1, wherein said information processor and RF output controller compares the temperature measured at the series of temperature sensors to a user-selected target temperature for ablation of cardiac tissue, and wherein said information processor and RF output controller limits the delivery of the electrical current through said electrical coupling to maintain the target temperature at the cardiac tissue.
 4. The system of claim 3, wherein each temperature sensor of the series of temperature sensors is adjacent to an electrode of the series of electrodes, and wherein the information processor and RF output controller utilizes a combined temperature reading from the temperature sensors on both sides of each electrode of the assembly of electrodes to independently control the delivery of current to each electrode.
 5. A system as recited in claim 1, wherein said information processor and RF output controller computes the elapsed time for the temperature measured at said temperature sensor to ramp up to a target temperature, the measured ramp up to the target temperature being representative of the power curve indicating power transferred to cardiac tissue, said information processor and RF output controller calculating the slope of the power curve for an ablation event from the elapsed time and the target temperature for determining the likelihood of coagulum formation.
 6. A system as recited in claim 5, wherein said information processor and RF output controller computes an index indicative of the likelihood of coagulum by dividing the slope of the power curve by the square of the electrical current delivered through the ablation electrode.
 7. A system as recited in claim 1, wherein the system further comprises a multiplicity of current and voltage sensors and the information processor and RF output controller comprises functionality for terminating delivery of RF energy to the series of ablation electrodes by comparing to maximum set points, real-time measurements of at least one of impedance at the ablation site, differential impedance at the ablation site, and temperature at the ablation site.
 8. The system as recited in claim 7, wherein the functionality utilizes analog methods for information processing and pulse width modulation for RF energy control.
 9. A system as recited in claim 8, wherein said information processor and RF output controller computes a Coagulum Index associated with capacitive properties of the ablation of cardiac tissue as proportional to the index indicating the likelihood of coagulum formation.
 10. A system as recited in claim 9, wherein the Coagulum Index of the ablation catheter is provided for matching the impedance determined for the ablation of cardiac tissue.
 11. A system as recited in claim 1, wherein delivery of electrical current is limited based on temperature measurements using analog methods for information processing and pulse width modulation for RF energy control.
 12. A method for forming a cardiac lesion by delivering radio frequency (RF) energy from an RF generator to an ablation site of cardiac tissue using an ablation catheter with an ablation electrode, said method comprising: a) selecting a temperature set point for the ablation site; b) applying the ablation catheter to the ablation site to establish contact between the ablation electrode and the ablation site, c) monitoring for effective contact between the ablation electrode and the ablation site by monitoring ablation site temperature, and optionally at least one of impedance at the ablation site, power of the RF generator, and current through the ablation site; d) initiating and gradually increasing power of the RF generator in a ramp-up phase to increase the temperature of tissue at the ablation site, said ramp-up phase terminating when temperature at the ablation site reaches a temperature of about the temperature set point; and e) maintaining the temperature at the ablation site at about the temperature set point by regulating the power of the RF generator, said maintaining terminating after forming a cardiac lesion, said initiating and increasing step and said maintaining step being regulated automatically and being terminated prematurely if effective contact is not present, thereby reducing coagulum formation.
 13. The method of claim 12, wherein the maximum power of the RF generator is set at or below about 35 watts and temperature is regulated to remain within 5° C. or less from the temperature set point.
 14. The method of claim 12, wherein the power of the RF generator is set at between about 7 W to about 15 W and the temperature is regulated to remain within 5° C. or less from the temperature set point.
 15. The method of claim 12, wherein the Coagulum Index is at or below a value of about
 12. 16. The method of claim 12, wherein the Coagulum Index is at or below a value of about
 8. 17. The method of claim 12, wherein the effective contact between the ablation electrode and the ablation site is determined by real-time measurement of at least one of impedance at the ablation site, differential impedance at the ablation site, and temperature at the ablation site.
 18. The method of claim 12, wherein the ablation electrode is a multiplicity of ablation electrodes and maintaining the temperature is regulated using temperature feedback from a multiplicity of thermocouple sensors positioned between the ablation electrodes of the assembly of ablation electrodes.
 19. The method of claim 18, wherein the maintaining the temperature is performed independently for each electrode by comparing the temperature of neighboring thermocouple sensors for each electrode.
 20. The method of claim 18, wherein the maintaining the temperature is performed by electronically subtracting the temperature of 2 or more neighboring thermocouple sensors and using the result to control the pulsewidth duration of the pulsewidth modulator.
 21. The method of claim 12, wherein the temperature set point is selectable by a user.
 22. The method of claim 12, wherein the configuration of simultaneous RF energy delivery through the multiplicity of linear or curvilinear ablation electrodes is selected by a user. 